The IOB format (short for inside, outside, beginning) is a common tagging format for tagging tokens in a chunking task in computational linguistics (ex. named-entity recognition).[1] It was presented by Ramshaw and Marcus in their paper "Text Chunking using Transformation-Based Learning", 1995[2] The B- prefix before a tag indicates that the tag is the beginning of a chunk, and an I- prefix before a tag indicates that the tag is inside a chunk. The B- tag is used only when a tag is followed by a tag of the same type without O tokens between them. An O tag indicates that a token belongs to no chunk.

Another similar format which is widely used is IOB2 format, which is the same as the IOB format except that the B- tag is used in the beginning of every chunk (i.e. all chunks start with the B- tag).

A readable introduction to entity tagging is given in Bob Carpenter's blog post, "Coding Chunkers as Taggers".[3] 'BIO' is plausibly a synonym for 'IOB'.

An example with IOB format:

Alex I-PER
is O
going O
to O
Los I-LOC
Angeles I-LOC

An example with IOB2 format:

Alex B-PER
is O
going O
to O
Los B-LOC
Angeles I-LOC

Related tagging schemes sometimes include "START/END: This consists of the tags B, E, I, S or O where S is used to represent a chunk containing a single token. Chunks of length greater than or equal to two always start with the B tag and end with the E tag."[4]